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Register a new userNew predictions for radiation-driven, steady-state mass-loss and wind-momentum from hot, massive stars. I. Method and first results
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[Abridged] Context: Radiation-driven mass loss plays a key role in the life-cycles of massive stars. However, basic predictions of such mass loss still suffer from significant quantitative uncertainties. Aims: We develop new radiation-driven, steady-state wind models for massive stars with hot surfaces, suitable for quantitative predictions of global parameters like mass-loss and wind-momentum rates. Methods: The simulations presented here are based on a self-consistent, iterative grid-solution to the spherically symmetric, steady-state equation of motion, using full NLTE radiative transfer solutions in the co-moving frame to derive the radiative acceleration. We do not rely on any distribution functions or parametrization for computation of the line force responsible for the wind driving. Results: In this first paper, we present models representing two prototypical O-stars in the Galaxy, one with a higher stellar mass M/Msun=59 and luminosity log10(L/Lsun)= 5.87 and one with a lower M/Msun= 27 and log10(L/Lsun)= 5.1. For these simulations, basic predictions for global mass-loss rates and velocity laws are given, and the influence from additional parameters like wind clumping and microturbulent speeds discussed. A key result is that our mass-loss rates are significantly lower than those predicted by the mass-loss recipes normally included in models of massive-star evolution. Conclusions: Our results support previous suggestions that Galactic O-star mass-loss rates may be overestimated in present-day stellar evolution models, and that new rates thus might be needed. Indeed, future papers in this series will incorporate our new models into such simulations of stellar evolution, extending the very first simulations presented here toward larger grids covering a range of metallicities, B supergiants across the bistability jump, and possibly also Wolf-Rayet stars.
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Reliable predictions of mass-loss rates are important for massive-star evolution computations. We aim to provide predictions for mass-loss rates and wind-momentum rates of O-type stars, carefully studying the behaviour of these winds as functions of stellar parameters like luminosity and metallicity. We use newly developed steady-state models of radiation-driven winds to compute the global properties of a grid of O-stars. The self-consistent models are calculated by means of an iterative solution to the equation of motion using full NLTE radiative transfer in the co-moving frame to compute the radiative acceleration. In order to study winds in different galactic environments, the grid covers main-sequence stars, giants and supergiants in the Galaxy and both Magellanic Clouds. We find a strong dependence of mass-loss on both luminosity and metallicity. Mean values across the grid are $dot{M}sim L_{ast}^{2.2}$ and $dot{M}sim Z_{ast}^{0.95}$, however we also find a somewhat stronger dependence on metallicity for lower luminosities. Similarly, the mass loss-luminosity relation is somewhat steeper for the SMC than for the Galaxy. In addition, the computed rates are systematically lower (by a factor 2 and more) than those commonly used in stellar-evolution calculations. Overall, our results agree well with observations in the Galaxy that account properly for wind-clumping, with empirical $dot{M}$ vs. $Z_ast$ scaling relations, and with observations of O-dwarfs in the SMC. Our results provide simple fit relations for mass-loss rates and wind momenta of massive O-stars stars as functions of luminosity and metallicity, valid in the range $T_{rm eff} = 28000 - 45000$,K. Due to the systematically lower $dot{M}$, our new models suggest that new rates might be needed in evolution simulations of massive stars.
Massive stars and supernovae (SNe) have a huge impact on their environment. Despite their importance, a comprehensive knowledge of which massive stars produce which SNe is hitherto lacking. We use a Monte Carlo method to predict the mass-loss rates of massive stars in the Hertzsprung-Russell Diagram (HRD) covering all phases from the OB main sequence, the unstable Luminous Blue Variable (LBV) stage, to the final Wolf-Rayet (WR) phase. Although WR produce their own metals, a strong dependence of the mass-loss rate on the initial iron abundance is found at sub-solar metallicities (1/10 -- 1/100 solar). This may present a viable mechanism to prevent the loss of angular momentum by stellar winds, which could inhibit GRBs occurring at solar metallicities -- providing a significant boost to the collapsar model. Furthermore, we discuss recently reported quasi-sinusoidal modulations in the radio lightcurves of SNe 2001ig and 2003bg. We show that both the sinusoidal behaviour and the recurrence timescale of these modulations are consistent with the predicted mass-loss behaviour of LBVs. We discuss potential ramifications for the ``Conti scenario for massive star evolution.
We set out to determine stellar labels from low-resolution survey spectra of hot, OBA stars with effective temperature (Teff) higher than 7500K. This fills a gap in the scientific analysis of large spectroscopic stellar surveys such as LAMOST, which offers spectra for millions of stars at R=1800. We first explore the theoretical information content of such spectra for determining stellar labels, via the Cramer-Rao bound. We show that in the limit of perfect model spectra and observed spectra with S/N of 100, precise estimates are possible for a wide range of stellar labels: not only the effective temperature Teff, surface gravity logg, and projected rotation velocity vsini, but also the micro-turbulence velocity, Helium abundance and the elemental abundances [C/H], [N/H], [O/H], [Si/H], [S/H], and [Fe/H]. Our analysis illustrates that the temperature regime of around 9500K is challenging, as the dominant Balmer and Paschen line strength vary little with Teff. We implement the simultaneous fitting of these 11 stellar labels to LAMOST hot-star spectra using the Payne approach, drawing on Kuruczs ATLAS12/SYNTHE LTE spectra as the underlying models. We then obtain stellar parameter estimates for a sample of about 330,000 hot stars with LAMOST spectra, an increase by about two orders of magnitude in sample size. Among them, about 260,000 have good Gaia parallaxes (S/N>5), and more than 95 percent of them are luminous stars, mostly on the main sequence; the rest reflects lower luminosity evolved stars, such as hot subdwarfs and white dwarfs. We show that the fidelity of the abundance estimates is limited by the systematics of the underlying models, as they do not account for NLTE effects. Finally, we show the detailed distribution of vsini of stars with 8000-15,000K, illustrating that it extends to a sharp cut-off at the critical rotation velocity, across a wide range of temperatures.
We discuss the role of mass loss for the evolution of the most massive stars, highlighting the role of the predicted bi-stability jump that might be relevant for the evolution of rotational velocities during or just after the main sequence. This mechanism is also proposed as an explanation for the mass-loss variations seen in the winds from Luminous Blue Variables (LBVs). These might be relevant for the quasi-sinusoidal modulations seen in a number of recent transitional supernovae (SNe), as well as for the double-throughed absorption profile recently discovered in the Halpha line of SN 2005gj. Finally, we discuss the role of metallicity via the Z-dependent character of their winds, during both the initial and final (Wolf-Rayet) phases of evolution, with implications for the angular momentum evolution of the progenitor stars of long gamma-ray bursts (GRBs).
The rate at which massive stars eject mass in stellar winds significantly influences their evolutionary path. Cosmic rates of nucleosynthesis, explosive stellar phenomena, and compact object genesis depend on this poorly known facet of stellar evolution. We employ an unexploited observational technique for measuring the mass-loss rates of O- and early-B stars. Our approach, which has no adjustable parameters, uses the principle of pressure equilibrium between the stellar wind and the ambient interstellar medium for a high-velocity star generating an infrared bowshock nebula. Results for twenty bowshock-generating stars show good agreement with two sets of theoretical predictions for O5--O9.5 main-sequence stars, yielding $dot M=$1.3$times$10$^{-6}$ to 2$times$10$^{-9}$ solar masses per year. Although $dot M$ values derived for this sample are smaller than theoretical expectations by a factor of about two, this discrepancy is greatly reduced compared to canonical mass-loss methods. Bowshock-derived mass-loss rates are factors of ten smaller than H$alpha$-based measurements (uncorrected for clumping) for similar stellar types and are nearly an order of magnitude larger than P$^{4+}$ and some other UV absorption-line-based diagnostics. Ambient interstellar densities of at least several cm$^{-3}$ appear to be required for formation of a prominent infrared bowshock nebula. $dot M$ measurements for early-B stars are not yet compelling owing to the small number in our sample and the lack of clear theoretical predictions in the regime of lower stellar luminosities. These results may constitute a partial resolution of the extant weak-wind problem for late-O stars. The technique shows promise for determining mass-loss rates in the weak-wind regime.
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